Study aim: Comparison between the effectiveness of letrozole and hormone replacement therapy in frozen embryo transfer technique in polycystic ovarian syndrome (PCOS) Design: This is a randomized controlled trial on 130 infertile women with polycystic ovary syndrome (PCOS) who underwent invitro fertilization (IVF) or itracytoplasmic sperm injection (ICSI) and frozen embryo transfer. The eligible patients will be randomly allocated into two groups: stimulated cycle with letrozole and human menopausal gonadotropin (HMG; intervention group) and routine artificial hormonal endometrial preparation (control group). The permuted block randomization will be conducted according to a computer-generated list. The patients’ enrolment and assignment to groups will be carried out by a researcher midwife in the clinic. Each patient will participate in the study only once and if she has written consent. Settings and conduct: The study is conducted in Arash Womens Hospital in Baghdarnia St., Tehran, Iran. PCOS diagnosis is based on the Rotterdam criteria. The physician, the researcher midwife and the patient all are aware of received treatment. Participants/Inclusion and exclusion criteria: Women with polycystic ovarian syndrome and infertility who are candidate for frozen embryo transfer Inclusion criteria: 1. Age 20-35 years; 2. First or second embryo transfer cycle; 3. Normal uterine cavity Exclusion criteria: 1. Severe male factor; 2. pregestational diagnosis; 3. Severe endometriosis; 4. Concurrent immune disease; 5.Follicle stimulating hormone level > 10 IU/ml; 6. Body mass index >= 30 Kg/m2 Intervention groups: Patients will randomly selected in two groups to receive either (i) estradiol valerate or (ii) concurrent administration of letrozole and Human Menopausal Gonadotropine (HMG) for ovulation induction Main outcome variables: Number (ratio) of successful pregnany Number (ratio) of abortion